Julie LeFever Introduction The study of fossilized tracks, trails, tubes and burrows associated 1) Tracks — impressions left by a moving individual’s appendage with organisms that burrow into and live in, or near the bottom (walking or running); deposits of an ocean is called Ichnology. First studies of trace 2) Trails — continuous groove produced during locomotion fossils date from 1823 to 1881 when they were originally assigned (sliding or slithering); to the plant kingdom as fossil algae, “fucoids” (Osgood, 1975). 3) Burrows — more or less permanent structures excavated During those early examinations, the gross morphology of the within the sediment; and, “fucoids” was thought to resemble plants thus supporting the 4) Bioerosion structures — organisms mechanically or assignment. In 1880, evaluation of previous work raised questions biochemically boring into a rigid substrate (Angulo and of validity. Many “fucoids” were determined to be traces of Buatois, 2009). invertebrate activity and others were sedimentary structures. Discussions continued to 1927, when “fucoids” were disproved There is a great deal of difficulty in relating the traces back to the and the study of trace fossils all but ceased. Little progress was trace maker. The trace maker is rarely, if ever, found in or near the made in the study of trace fossils from 1927 to 1960. Three trace. Understanding is gained by interpreting the behavior that reasons are cited for this lack of progress: 1) problems related led to the formation of the trace, was it resting, crawling, grazing, to taxonomy (the classification of organisms), 2) there was no feeding, dwelling, or escaping. These behavioral responses may statement as to the value of trace fossil studies to paleontology, also be related to environmental changes in salinity, oxygenation, zoology, and sedimentology, and 3) there was limited interest temperature and water turbidity (Gingras et al. 2009). Features by sedimentologists and paleontologists in paleoecology (the that indicate the environmental changes listed above include: interaction between ancient organisms and their environment) and the use of trace fossils to help determine past environments. 1) Unburrowed – indicator of fresh water Since that time a classification of trace fossils has been developed 2) Distribution of the types of traces using sedimentological and behavior methods. Trace fossils have 3) Size of the trace – for example, the size of the trace in a proven useful in many aspects of geology including facies analysis, marine environment would be an indicator of oxygen content animal behavior, and morphology. This paper is intended to be an and food supplies; small traces might indicate a stress introduction to trace fossils, not an extensive report with all the environment; and high diversity would result from abundant details. food, oxygen, and normal salinities.
Trace Fossils versus Body Fossils This leads to a simple, practical classification based on the Trace fossils represent behavior or activity by organisms rather morphology of their trace (fig. 1). There is not a direct correlation than the actual body parts of the organism itself (Frey, 1975). between ichnofacies and depositional environments; instead they Trace fossils (“facies fossils”) are significantly different from represent a set of environmental factors (Angulo and Buatois, body fossils in that they extend over longer periods of time 2009). These can be split into 4 main categories (fig.2): and have more restricted geographical distribution. Particular assemblages of trace fossils reflect particular environments of 1) Softground marine - Psilonichus, Skolithos, Cruziana, deposition thus making them very useful for environmental Zoophycos, Nereities reconstruction and extending those environments to strata of 2) Substrate-controlled – Glossifungites, Trypanites, Teredollites significantly different ages. The traces are not necessarily formed 3) Continental invertebrate penecontemporaneously with sedimentation, their presence 4) Vertebrate indicating the sediment had become more stable allowing for mobile feeders. The abundance of different types of trace Subsurface studies in the Williston Basin focus on the top two fossils appears to be linked with energy conditions at the time of categories, the softground marine and the substrate controlled. sedimentation (larger diversity is associated with quieter waters). These are the traces that are encountered when working with cores from the basin (fig. 3). Body fossils comprise the primary record of ancient life. Trace fossils record the expression of that life. Soft-bodied organisms are primarily represented by traces. Traces include:
6 GEO NEWS Ichnofabric Analysis Ichnofacies analysis starts by determining the discrete and the parameters (Angulo and Buatois, 2009). There are three factors poorly defined traces in cross-section view. Emphasis is placed that are the most important in controlling tiering. The first on cross-cutting relationships. This view allows for tiering, where changes the consistency of the sediment by compaction due to there is a vertical partitioning of the habitat due to changes in vertical accretion and progressive burial and dewatering. The the environment including physical, chemical, and biological second relies on the abundance of organic matter. The greatest amount of organic matter is at the sediment-water interface. The number of organisms present can be directly related to the abundance of food. The number of feeders decreases away from the food source. The third factor shows a similar trend regarding the oxygen content of the sediment. Sorting all of the trace fossils into a tiering structure can be difficult especially in areas of intense bioturbation.
To assist in the analysis of trace fossils, the degree of bioturbation is assigned a number (bioturbation index) that represents the percent of the sediment that is disturbed (table 1, fig. 4). This can be plotted visually alongside the tiering diagram and other parts of the analysis to aid in the determination of the sedimentary environments (fig. 5).
A collection of the information can then be used to determine the sedimentary environment. Gingras et al. (2009) developed a flow chart (fig. 6) that aids in the interpretation of common bioturbate textures.
Figure 1. Three dimensional diagram displaying morphological classifi- cation of burrows and their associated names (from Gingras et al. 2009)
Figure 2. Ichnofauna model of the soft- ground and substrate controlled trace fossil assemblages and how they relate to marine environment. Each is comprised of a series of traces that reflect environmental factors which, in association with sedimentary and hard body fossils, as- sist in the determina- tion of the sedimen- tary environment. Diagram from Angulo and Buatois (2011).
JULY 2016 7 Figure 3. Example of Thalassinoides burrows from the Ordovician Red River Formation from the Terra Resources, Inc. – BNRR #1-17 (NWSW Sec. 17, T145N., R103W; API: 33-053-00955- 0000; depth 12,963.1 ft) well with an diagram- matic 3D representation of the boxwork of the burrows. Preferential dolomitization of the bur- row over the rock matrix causes the burrows to standout. It also comprises the reservoir for the formation. Diagram from Gerard and Bromley (2008).
A. B. C.
B.I. - 0-1
B.I. 3 Figure 4. Examples of variations in Bioturbation Index within the Middle Member of the Bakken Formation in North Dakota. A and C are from the Whiting Oil and Gas Corp. – Braaflat #11-11H (NWNW Sec. 11, T153N, R91W; API #33-061-00641-0000; depths – 9,881 ft and 9,901 ft). The core sample that is shown in B is from the EOG Resources, Inc. – Fertile #1-12H (SESE Sec. 12, T151N, R90W; API #33-061-00557-0000; depth – 9,382 ft). Core sample A has limited burrowing with laminations that are virtually undisturbed. Core sample B is moderately bioturbated while still retaining some of the original laminations. Core sample C is extensively bioturbated. All of the sediment has been displaced by the burrows. Scale bar in white is one inch.
8 GEO NEWS Table 1. Criteria for the determination of the Bioturbation Index. See Figure 4 for the visual display of this type of data.
Figure 5. Example of a core log from Bakken well 15-31-03-11W2 showing the result of applying the ichnofabric approach. Illustration on the left represents the described core displaying the burrows and sedimentary structures. This is followed by the age, nomenclature, depth, bioturbation index, and ultimately the sedimentary facies and environments. Diagram from Angulo and Buatois, 2012.
JULY 2016 9 Figure 6. Flow chart developed by Gingras et al. (2009) demonstrating the process of interpreting ichnological fabrics.
Conclusions Trace fossils are an additional tool that assists the geologist in Buatois, L.A., and Mángano, M.G., 2011, Ichnology – Organism-substrate determining the environment of deposition. Multiple factors interactions in space and time: Cambridge University Press, 358 p. contribute to the absence or abundance of trace fossils in the Frey, R.W., 1975, Realm of Ichnology, in, Frey, R. W., ed., The Study of rocks. Attention to detail, in addition with hard body fossils Trace Fossils: A Synthesis of Principles, Problems, and Procedures in and sedimentary structures, add to our knowledge of past Ichnology: New York, Springer-Verlag, p. 13-38. environments. Gerard, J.R.F. and Bromley, R.G., 2008, Ichnofabrics in clastic sediments: Applications to sedimentological core studies: Madrid, J.R. F. Gerard References Cited Publishing, 97p. Angulo, S., and Buatois, L., 2009, Environmental setting of the Upper Gingras, M.K., Bann, K.L., MacEachern, J.A., Waldron, J., and Pemberton, Devonian-Lower Mississippian Bakken Formation of Subsurface S. George, 2009, A Conceptual Framework for the Application of Saskatchewan: Integrating Sedimentologic and Ichnologic data, A Trace Fossils: in, MacEachern, J.A., Bann, K.L., Gingras, M.K., and Core Workshop: in, 17th Williston Basin Petroleum Conference and Pemberton, S. George, eds, Applied Ichnology: SEPM Short Course Prospect Expo Core Workshop: Regina, SK, p. 66-163. No. 52., p. 1-26. Angulo, S., and Buatois, L., 2012, Ichnology of a Late Devonian-Early Osgood, R. G., Jr., 1975, The History of Invertebrate Ichnology, in, Frey, Carboniferous low-energy seaway: The Bakken Formation of R. W., ed., The Study of Trace Fossils: A Synthesis of Principles, subsurface Saskatchewan, Canada: Assessing paleoenvironmental Problems, and Procedures in Ichnology: New York, Springer-Verlag, controls and biotic responses: Paleogeography, Paleoclimatology, p. 3-10. Paleoecology, v. 315-316, p. 46-60.
10 GEO NEWS